1. Field of the Invention
In the process of forming circuit patterns, by lithography, on a semiconductor substrate or on the substrate of a liquid crystal display, a cleaning step is performed to clean the substrate and remove, from the substrate, residues such as organic substances, metal impurities, particles and resist.
2. Description of the Related Art
In the cleaning step, the substrate is first treated with a process solution, whereby residue is removed from the substrate. Then, the substrate is cleaned with pure water or cleaning solution, removing the residues and the process solution from the substrate.
Suitable as the process solution is a solution containing an oxidizing agent such as peroxomonosulfuric acid or peroxodisulfuric acid. As is known in the art, such a solution may be obtained by adding hydrogen peroxide solution to sulfuric acid.
When hydrogen peroxide is mixed with sulfuric acid to generate the process solution, however, the concentration of the sulfuric acid may greatly decrease because the hydrogen peroxide solution is, for example, a 35% aqueous solution. If the process solution is repeatedly recovered and used again, each time mixed with a newly generated process solution, its concentration will decrease and its processing ability will be greatly reduced.
In view of this, a method of electrolyzing sulfuric acid without using hydrogen peroxide solution, thereby to generate a solution that contains an oxidizing agent has been put to practical use. Such a method is disclosed in Patent Document 1 identified below.
The apparatus for performing electrolysis, disclosed in Patent Document 1, has an electrolytic cell. The electrolytic cell is separated into an anode chamber and a cathode chamber. An anode and a cathode are provided in the anode chamber and the cathode chamber, respectively. Sulfuric acid is introduced into both the anode chamber and the cathode chamber. The sulfuric acid is subjected to electrolysis in the anode chamber, generating a solution containing an oxidizing agent. The solution thus generated is used as a process solution.    [Patent Doc. 1] Jpn. Pat. Appln. KOKAI Publication No. 2001-192874
If sulfuric acid is electrolyze, generating a process solution, as disclosed in Patent Document 1, the concentration of the process solution will scarcely decrease if repeatedly used, each time mixed with a newly generated process solution. The process solution can therefore maintain its processing ability.
In the apparatus disclosed in Patent Document 1, however, sulfuric acid is supplied to the anode chamber and the cathode chamber through different supply systems. That is, a sulfuric acid tank and a supply pump, which constitute one supply system, are connected to the anode chamber, and a sulfuric acid tank and a supply pump, which constitute another supply system, are connected to the cathode chamber. Since two supply systems are indispensable, the apparatus is inevitably complicated and large.